1. Field of the Invention
The present invention relates to a scanning unit for a position measuring system for the optical scanning of a scale, having a light source for transmitting light in the direction toward a section of the scale, a detector for receiving the light modified by the scale, and a lens arrangement placed in front of the detector. The present invention further relates to a position measuring system utilizing such a scanning unit.
2. Discussion of Related Art
A scanning unit may include a light source for transmitting light in the direction of the scale which is provided with a track which can be optically scanned, a detector for receiving the light which was modified, for example reflected, by the scale, as well as a lens arrangement having a plurality of lenses and arranged in front of the detector for generating on the detector a defined image of an area scanned by the light. The scanned area can for one be a scanned area on the scale, or an intermediate image in the scanning light beam.
Such a scanning unit can be provided, for example, for scanning a scale provided with an incremental graduation by the incident or transmitted light method. In the first mentioned case, the light transmitted by the light source in the direction of the scale is modified by the scale and reflected. In the second case mentioned, the light transmitted by the light source passes through the (partially transparent) scale and is modified in the process. Such a scanning unit, including a position measuring device, is known from DE 103 17 736 A1.
Optical imaging, or projection, of the scanning area scanned on the scale takes place via the two part lens arrangement in the beam path at different possible optical imaging magnifications β=1, or β>1, or β<1, onto an area identical in regard to the surface measure, or onto an image field identical in regard to the surface measure in the detector plane.
The surface, or the area, remains of the same size on the part of the object and the image in all cases of the resultant picture. This means that the scanned scanning area, including the information contained therein, is transmitted to an image field of the same size on the detector. Thus, in connection with such systems there is the basic requirement for as large as possible a scanned area. In the case of a scanning area on the scale, the entire system is possibly less sensitive to soiling or contamination. In the case of optical imaging a scanning area of the same size of the image and the object, definite minimally required detector areas result, which cannot be decreased without loss of information.
However, the detector area is a basically critical value in connection with such scanning units, in particular if, for example, CMOS photodiodes are employed as detectors. For one, because the available area can be very expensive, depending on the respectively used manufacturing process, and also for the reason that with an increasing detector area the capacity of the photodiodes also increases, which in turn results in a large proportion of noise in the signal to be detected.
In the case of using a lens arrangement as known from DE 103 17 736 A1, at an image magnification of β=1 the information regarding the scanned periodic incremental graduation is retained as mentioned in the generated image in the detection plane when the scanned area and the generated image have the same area. However, in the case of realized scanning magnification of β≠1, when using such lens arrangements the correct information regarding the scanned periodic incremental graduation does not result in the image on the detector side. For illustrating this problem, the image conditions in regard to a scanned incremental graduation of the image magnifications β<1 (FIG. 1a) and β>1 (FIG. 1b) are shown in FIGS. 1a and 1b. Here, a section of an incremental graduation MINC is respectively pictured by a known two-part lens arrangement L, wherein the section A corresponds to the scanning area on the part of the object. Following the optical imaging of a portion of this section via a first single lens L1 of the lens arrangement L, an intermediate image ZB results, this is optically imaged via a second assigned single lens L2 as an image in the image or detection plane D, while maintaining the size of the image field B, i.e. A=B. This takes place analogously by each individual pair of lenses of the lens arrangement L.
In the case of FIG. 1a with an image magnification β<1, a reduction of the size of a section A of the incremental graduation MINC in the detection plane D takes place per image. As a whole, an image of the incremental graduation MINC of the same size as the scanned section A results in the image field B. However, the periodicity of the scanned incremental graduation MINC is no longer correctly represented in the image, as can be seen in FIG. 1a. 
In the case of FIG. 1b with an image magnification β>1, an image results in the detection plane D from the scanning of the section A of the incremental graduation MINC, which results from an undefined superimposition of individual partial images. The periodicity of the scanned incremental graduation MINC is no longer correctly represented in the image.
Similar problems exist of course if, for example with a different scanning principle, an intermediate image is to be optically imaged in an image field on the detector instead of a scanned area on the scale.